1. Field of the Disclosure
The disclosure generally relates to methods and catalysts for the epoxidation of alkenes and, more specifically, to Ag(100) catalysts for the epoxidation of ethylene.
2. Brief Description of Related Technology
The domestic production of ethylene oxide, by the direct epoxidation of ethylene over silver (Ag) catalysts, yields about 10 billion pounds of ethylene oxide per year, valued in excess of five billion dollars. Ethylene oxide is ordinarily used in the production of ethylene glycol (antifreeze), as well as detergents, ethanolamines, lubricants and many other commodity products.
The prior art has sought to develop increasingly selective catalysts for epoxide formation by, for example, modifying traditional silver (Ag) catalysts with a variety of promoters. U.S. Pat. Nos. 6,750,173, 5,905,053, and 5,663,385 describe the use of promoters to improve the selectivity of ethylene epoxidation catalysts. These patents generally teach adding alkali metals, alkali earth metals, transition metals, main group elements, and compositions containing the same to silver to improve the selectivity of the ethylene-to-ethylene oxide reaction.
Other prior art demonstrates that the reactivity of silver epoxidation catalysts and promoted-silver epoxidation catalysts remains unchanged regardless of which of Ag(111) and Ag(110) crystallographic planes predominate on the catalyst. Cambell (1985) J. Catal. 94: 436-444. Furthermore, structural differences in the silver epoxidation catalyst have been found to produce little to no effect on the selectivity and reactivity of the catalyst. Sajkowski et al. (1987) Catal. Rev. Sci. Eng. 29: 325-360.
More recently, the prior art suggests that crystal step edges are preferential to surface planes as epoxidation reaction sites. Briefly, a crystal is built from layers of atoms in a given order, the exterior of these crystals can be a plane of well-ordered atoms or molecules or can be a plane that bisects the well-ordered planes within the crystal. If the exterior of the crystal bisects ordered planes within a crystal then the transition between one ordered plane and the next is referred to as a step edge, analogous to a step edge on a flight of stairs wherein each stair is a well ordered plane of atoms. For example, the prior art suggests that step edges are more reactive than the silver surface planes for silver Ag(100) crystals. Kokalj et al, Adsorption of Ethylene on Stepped Ag(n10) Surfaces (Sep. 8, 2003) (Conference presentation at the 22nd European Conference On Surface Science) available at http://www.fzu.cz/activities/conferences/ecoss22/abstracts/17005.pdf; see also Massa, Crystal Structure Determination 2nd ed., Springer, (2004), 20-22 (defining Miller indices and the surface plane notation). Correspondingly, in light of the catalytic reaction's insensitivity to crystallographic planes and the higher reactivity of step edges, one skilled in the art would understand that higher indices, e.g., Ag(410), would provide a better epoxidation catalyst because of the increase number of step edges.
Generally, the prior art neither sufficiently teaches nor suggests to one of ordinary skill in the art the use of Ag(100) as an epoxidation catalyst nor alkene epoxidation over Ag(100).